Semiconductor device and fabrication method thereof

ABSTRACT

A method for fabricating a semiconductor device includes forming an isolation feature in a substrate, forming a gate stack over the substrate, forming a source/drain (S/D) recess cavity in the substrate, where the S/D recess cavity is positioned between the gate stack and the isolation feature. The method further includes forming an epitaxial (epi) material in the S/D recess cavity, where the epi material has an upper surface which including a first crystal plane. Additionally, the method includes performing a redistribution process to the epi material in the S/D recess cavity using a chlorine-containing gas, where the first crystal plane is transformed to a second crystal plane after the redistribution.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No. 13/248,319, filed Sep. 29, 2011, the disclosure of which is incorporated herein by reference in its entirety.

FIELD

The disclosure relates to integrated circuit fabrication and, more particularly, to a semiconductor device with a strained structure.

BACKGROUND

When a semiconductor device, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), is scaled down through various technology nodes, high-k gate dielectric layers and metal gate electrode layers are incorporated into the gate stack of the MOSFET to improve device performance with the decreased feature sizes. In addition, strained structures in source and drain (S/D) recess cavities of the MOSFET utilizing selectively grown silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implement such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. As the gate length and spacing between devices decrease, these problems are exacerbated. For example, it is difficult to achieve enhanced carrier mobility for a semiconductor device, because strained materials cannot deliver a given amount of strain into the channel region of the semiconductor device, thereby increasing the likelihood of device instability and/or device failure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the relative dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method for fabricating a semiconductor device comprising a strained structure according to various aspects of the present disclosure; and

FIGS. 2-8 show schematic cross-sectional views of a strained structure of a semiconductor device at various stages of fabrication according to various aspects of the present disclosure.

DESCRIPTION

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

FIG. 1 is a flowchart illustrating a method 100 for fabricating a semiconductor device 200 according to various aspects of the present disclosure. FIGS. 2-8 show schematic cross-sectional views of a semiconductor device 200 at various stages of fabrication according to an embodiment of the method 100 of FIG. 1. The semiconductor device 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 1 does not produce a completed semiconductor device 200. A completed semiconductor device 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 1 through 8 are simplified for a better understanding of the present disclosure. For example, although the figures illustrate the semiconductor device 200, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 wherein a substrate 202 comprising a surface 202 s is provided. In one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). In the present embodiment, the substrate 202 is referred to as a (100) substrate having the surface 202 s formed of the (100) crystal plane. In an alternative embodiment, the substrate 202 may include a silicon-on-insulator (SOI) structure.

The substrate 202 may further comprise active regions 204. The active regions 204 may include various doping configurations depending on design requirements. In some embodiments, the active regions 204 may be doped with p-type or n-type dopants. For example, the active regions 204 may be doped with p-type dopants, using a chemical such as boron or BF₂ to perform the doping; n-type dopants, using a chemical such as phosphorus or arsenic to perform the doping; and/or combinations thereof. The active regions 204 may act as regions configured for a N-type metal-oxide-semiconductor transistor device (referred to as an NMOS) and regions configured for a P-type metal-oxide-semiconductor transistor device (referred to as a PMOS).

In some embodiments, isolation structures 206 a and 206 b are formed in the substrate 202 to isolate the various active regions 204. The isolation structures 206 a and 206 b, for example, are formed using isolation technology, such as local oxidation of silicon (LOCOS) or shallow trench isolation (STI), to define and electrically isolate the various active regions 204. In the present embodiment, the isolation structures 206 a and 206 b include a STI. The isolation structures 206 a and 206 b may comprise silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-K dielectric material, other suitable materials, and/or combinations thereof. The isolation structures 206 a and 206 b, and in the present embodiment, the STI, may be formed by any suitable process. As one example, the formation of the STI may include patterning the semiconductor substrate 202 by a photolithography process, etching a trench in the substrate 202 (for example, by using a dry etching, wet etching, and/or plasma etching process), and filling the trench (for example, by using a chemical vapor deposition process) with a dielectric material. In some embodiments, the filled trench may have a multi-layer structure such as a thermal oxide liner layer filled with silicon nitride or silicon oxide.

Still referring to FIG. 2, in at least one embodiment, gate stacks 210 a, 210 b, and 210 c are formed over the surface 202 s of the substrate 202. In some embodiments, the gate stacks 210 a, 210 b, and 210 c are formed by sequentially depositing and patterning a gate dielectric layer 212, a gate electrode layer 214, and a hard mask layer 216 on the substrate 202.

The gate dielectric layer 212, in one example, is a thin film comprising silicon oxide, silicon nitride, silicon oxynitride, high-k dielectrics, other suitable dielectric materials, or combinations thereof. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gate dielectric layer 212 is a high-k dielectric layer with a thickness in the range of about 10 angstroms to about 30 angstroms. The gate dielectric layer 212 may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric layer 212 may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric layer 212 and substrate 202. The interfacial layer may comprise silicon oxide.

The gate electrode layer 214 is then formed on the gate dielectric layer 212. In some embodiments, the gate electrode layer 214 may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layer 214 may comprise polysilicon. Further, the gate electrode layer 214 may be doped polysilicon with the same or different doping species. In one embodiment, the gate electrode layer 214 has a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 214 may be formed using a process such as low-pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), other suitable processes, or combinations thereof.

Next, the hard mask layer 216 is formed over the gate electrode layer 214 and a patterned photo-sensitive layer (not shown) is formed on the hard mask layer 216. The pattern of the photo-sensitive layer is transferred to the hard mask layer 216 and then transferred to the gate electrode layer 214 and gate dielectric layer 212 to form the gate stacks 210 a, 210 b, and 210 c over the surface 202 s of the substrate 202. In some embodiments, the hard mask layer 216 comprises silicon oxide. Alternatively, the hard mask layer 216 may comprise silicon nitride, silicon oxynitride, and/or other suitable dielectric materials, and may be formed using a method such as CVD or PVD. The hard mask layer 216 has a thickness in the range from about 100 angstroms to about 800 angstroms. The photo-sensitive layer is stripped thereafter by a dry and/or wet stripping process.

Referring to FIGS. 1 and 3, the method 100 proceeds to step 104 wherein gate spacers 218 are formed overlying opposite sidewalls of the gate stacks 210 a, 210 b, and 210 c. In the present embodiment, the gate spacers 218 adjoin sidewalls of the gate stacks 210 a, 210 b. In some embodiments, the gate spacers 218 may include a single-layer or a multiple-layer structure. In the present embodiment, a blanket layer of spacer material (not shown) is formed over the gate stacks 210 a, 210 b, and 210 c by a deposition process including CVD, PVD, ALD, or other suitable techniques. In some embodiments, the spacer material comprises silicon oxide, silicon nitride, silicon oxynitride, other suitable material, or combinations thereof. In some embodiments, the spacer material has a thickness ranging from about 5 nm to about 15 nm. Then, an anisotropic etching is performed on the spacer material to form the gate spacers 218.

Referring to FIGS. 1 and 4, the method 100 continues with step 106 in which the substrate 202 is recessed to form recess cavities 220, 230, 240, and 250 in the substrate 202. In some embodiments, the recess cavities 220, 230, 240, and 250 are source and drain (S/D) recess cavities. In the structure of FIG. 4, the recess cavities 220 and 250 are formed between the gate stack 210 a/the isolation structure 206 a and the gate stack 210 c/the isolation structure 206 b, respectively. The recess cavities 230 and 240 are formed between the gate stacks 210 a/210 b and 210 b/210 c, respectively.

In the present embodiment, the processes for forming the recess cavities 220, 230, 240, and 250 are started using an isotropic dry etching process, followed by an anisotropic wet or dry etching process. In some embodiments, the isotropic dry etching process is performed using the gate spacers 218 and isolation structures 206 a and 206 b as hard masks to recess the surface 202 s of the substrate 202 that is unprotected by the gate spacers 218 or the isolation structures 206 a and 206 b to form initial recess cavities (not shown) in the substrate 202. In an embodiment, the isotropic dry etching process may be performed under a pressure of about 1 mTorr to about 1000 mTorr, a power of about 50 W to about 1000 W, a bias voltage of about 20 V to about 500 V, at a temperature of about 40° C. to about 60° C., using HBr and/or Cl₂ as etch gases. Also, in the embodiments provided, the bias voltage used in the isotropic dry etching process may be tuned to allow better control of an etching direction to achieve desired profiles for the S/D recess regions.

In some embodiments, a wet etching process is then provided to enlarge the initial recess cavities to form the recess cavities 220, 230, 240, and 250. In some embodiments, the wet etching process is performed using a chemical comprising hydration tetramethyl ammonium (TMAH), or the like. As a result of such etching processes, there may be formed a plurality of facets in each recess cavities 220, 230, 240, and 250. It should be noted that periphery environment with or without an etch stop can influence resultant features of the S/D recess cavities 220, 230, 240, and 250. During the wet etching process, the isolation structure 206 a may function as an etch stop for defining the recess cavity 220 between the gate stack 210 a and isolation structure 206 a. In some embodiments, the recess cavity 220 between the gate stack 210 a and isolation structure 206 a has respective sidewall surfaces defined by a bottom facet 220 c, an upper sidewall facet 220 a, lower sidewall facets 220 b and 220 d, and an upper portion of the sidewall of the isolation structure 206 a. Thereby, the facet 220 a and facet 220 b thus formed intersect each other and together define a wedge 220 w in the recess cavity 220, such that the wedge-shaped recess cavity 220 extends into the substrate 202 in the region right underneath the spacer 218 toward the channel region. In some embodiments, the recess cavity 230 between the adjacent gate stacks 210 a and 210 b without an etch stop have respective sidewall surfaces each defined by a bottom facet 230 c, upper sidewall facets 230 a and 230 e, and lower sidewall facets 230 b and 230 d. Thereby, the facet 230 d and facet 230 e thus formed intersect each other and together define a wedge 230 w in the recess cavity 230, such that the wedge-shaped recess cavity 230 extends into the substrate 202 in the region right underneath the spacer 218 toward the channel region.

In the illustrated example, the bottom facets 220 c, 230 c are formed of (100) crystal plane parallel to the crystal plane of the surface 202 s of the substrate 202. In the illustrated example, the upper sidewall facets 220 a, 230 a, and 230 e and the lower sidewall facets 220 b, 220 d, 230 b, and 230 d are formed of (111) crystal plane, and the upper sidewall facets 220 a and 230 a form an angle θ₁ to the bottom facets 220 c and 230 c. Further, the lower sidewall facets 220 b and 230 b form a smaller angle θ₂ than the angle θ₁ with respect to the bottom facets 220 c and 230 c. In the structure of FIG. 4, the angle θ₁ takes the range of about 90 degrees to about 150 degrees, while the angle θ₂ takes the range of about 40 degrees to about 60 degrees. In the present embodiment, the angles θ₁ and θ₂ take the values of 146 degrees and 56 degrees respectively in the case the facets 220 a, 230 a, 220 b, and 230 b are formed by the (111) crystal plane of the substrate 202. However, it should be noted that the structure of FIG. 4 is not limited to the case in which the facets 220 a, 230 a, 220 b, and 230 b are formed by the (111) crystal plane.

Further, the bottom facet 220 c is formed at a depth D₁ as measured from the surface 202 s of the substrate 202, while the upper facet 220 a is formed down to a depth D₂. In the structure of FIG. 4, the depth D₁ is in the range of about 20 nm to about 70 nm, while the depth D₂ is in the range of about 5 nm to about 60 nm. By optimizing the depth D₂ and a distance between the mutually opposing wedges 220 w, 230 w, it becomes possible to confine the uniaxial compressive stress of a strained material 222 (shown in FIG. 5) effectively to the channel region, thereby enhancing the device performance.

The process steps up to this point have provided the substrate 202 having the recess cavities 220, 230, 240, and 250 adjacent to the gate stacks 210 a, 210 b, and 210 c. Referring to FIGS. 1 and 5, the method 100 continues with step 108 in which a strained material 222 is grown in the recess cavities 220, 230, 240, and 250 of the substrate 202 using a process including selective epitaxy growth (SEG), cyclic deposition and etching (CDE), chemical vapor deposition (CVD) techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy (MBE), other suitable epi processes, or combinations thereof. In some embodiments, the strained material 222 has a lattice constant different from the substrate 202 to induce a strain or stress on the channel region of the semiconductor device 200, and therefore enable carrier mobility of the device to enhance the device performance.

In the present embodiment, a pre-cleaning process is performed to clean the recess cavities 220, 230, 240, and 250 using a chemical comprising hydrofluoric acid (HF) or other suitable solution. Then, gaseous and/or liquid precursors may be provided to interact with the composition of the substrate 202 to form the strained material 222, such as silicon germanium (SiGe), to fill the recess cavities 220, 230, 240, and 250. In one embodiment, the process for forming the strained material 222 comprising SiGe is performed at a temperature of about 600° to 750° C. and under a pressure of about 10 Torr to about 80 Torr, using reaction gases comprising SiH₂Cl₂, HCl, GeH₄, B₂H₆, H₂, or combinations thereof. In some embodiments, a ratio of a mass flow rate of the SiH₂Cl₂ to a mass flow rate of the HCl is in the range of about 0.45 to 0.55. In one embodiment, the strained material 222 is therefore grown from the facets 230 a, 230 b, 230 c, 230 d, and 230 e to the center of the recess cavity 230 in the substrate 202. In another embodiment, the strained material 222 is therefore grown from the facets 220 a, 220 b, 220 c, and 220 d to the center of the recess cavity 220 in the substrate 202.

In some embodiments, the growth of the strained material 222 in the recess cavity 230 that is not adjacent to the isolation structure 206 a is mainly along the facet 230 c and therefore has an upper surface 222 a formed of (100) crystal plane. In the present embodiment, the growth of the strained material 222 in the recess cavity 220 that is adjacent to the isolation structure 206 a is limited by the isolation structure 206 a because the isolation structure 206 a formed by dielectric with an amorphous structure fails to offer nucleation sites to grow an epitaxial material. In some embodiments, the growth of the strained material 222 in the recess cavity 220 tends to have an upper surface 222 b formed of (111) crystal plane with a stable surface energy. The strained material 222 in the recess cavity 220 has a lower sidewall surface 222 c formed over the lower sidewall facet 220 b and therefore is formed of (111) crystal plane. In some embodiments, the lower sidewall surface 222 c is parallel to the upper surface 222 b. It can be seen in FIG. 5 that the strained material 222 adjacent to the isolation structure 206 a occupies a small portion of the recess cavity 220.

In FIG. 5A, the semiconductor device 200 is enlarged for better understanding of the profile of the strained material 222 in the recess cavity 220. In the present embodiment, the strained material 222 in the recess cavity 220 has a corner 222 d adjacent to the edge of the gate stack 210 a and having a tip higher than the surface 202 s of the substrate 202. The corner 222 d has a height D3 measured from the surface 202 s to the tip of the corner 222 d. In some embodiments, the height D3 ranges between about 1 nm and about 10 nm. As shown in FIG. 5A, there are a normal line L1 perpendicular to the surface 202 s of the substrate 202, a normal line L2 perpendicular to the upper surface 222 b of the strained material 222 in the recess cavity 220, and an angle θ₃ between the normal line L1 and the normal line L2. As mentioned above, the substrate 202, for example, is a (100) substrate having the surface 202 s formed of the (100) crystal plane, and the upper surface 222 b is formed of (111) crystal plane. Therefore, in the present embodiment, the angle θ₃ is about 54 degrees.

Referring to FIGS. 1 and 6, the method 100 continues with step 110 in which a cap layer 224 is formed over the strained material 222. The cap layer 224 may function as a protection layer to prevent the underlying strained material 222 from being over-etched in a subsequent etching process. In the present embodiment, the cap layer 224 is formed by an epi growth process. The cap layer 224 over the strained material 222 in the recess cavity 230, which is not adjacent to the isolation structure 206 a, has a thickness D4. In some embodiments, the thickness D4 ranges between about 1 nm and about 5 nm. The cap layer 224 over the strained material 222 in the recess cavity 220 adjacent to the isolation structure 206 a has a sidewall 224 c contacting the isolation structure 206 a with a thickness D5. In some embodiments, a ratio of the thickness D4 over the thickness D5 ranges between about 1 and about 3. In some embodiments, the cap layer 224 over the strained material 222 in the recess cavities 230 may grow along the crystal orientation of the upper surface 222 a and has an upper surface 224 a formed of (100) crystal plane. In some embodiments, the cap layer 224 over the strained material 222 in the recess cavities 220 may grow along the crystal direction of the upper surface 222 b and has an upper surface 224 b formed of (111) crystal plane.

In some embodiments, the cap layer 224 comprises a material different from the strained material 222. In some embodiments, the cap layer 224 is a silicon-containing layer. In the present embodiment, the cap layer 224 is silicon. In some embodiments, the cap layer 224 is formed by a process including selective epitaxy growth (SEG), cyclic deposition and etching (CDE), chemical vapor deposition (CVD) techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy (MBE), other suitable epi processes, or combinations thereof. In the present embodiment, the cap layer 224 is formed by a process that is the same as the process for forming the strained material 222. In some embodiments, the cap layer 224 is continuously formed after forming the strained material 222 by changing the process conditions to be performed at a temperature of about 700° C. to about 800° C., under a pressure of about 10 Torr to about 50 Torr, and using a silicon-containing gas (e.g., SiH₂Cl₂) as reaction gas. In an alternative embodiment, B₂H₆ and/or H₂ are introduced with the silicon-containing gas for forming the cap layer 224.

Referring to FIGS. 1 and 7, the method 100 continues with step 112 in which an etching process is provided to the semiconductor device 200. In some embodiments, the etching process is in-situ performed in the equipment for forming the cap layer 224 (e.g., CVD equipment), whereby there is not vacuum break between the step for forming the cap layer 224 and the step for performing the etching process. In alternative embodiments, the cap layer 224 is omitted and the etching process is in-situ performed in the equipment for forming the strained material 222. In the present embodiment, the etching process is performed by introducing an etching gas over the cap layer 224 and the strained material 222 without providing a power source or generating plasma. In an embodiment, the etching gas is a chlorine-containing gas, such as Cl₂ and/or HCl. In an alternative embodiment, the etching process is performed using the etching gas and a carrier gas, e.g., H₂ and/or N₂. In the present embodiment, the etching gas has a flow rate ranging between about 50 sccm and about 300 sccm, and the carrier gas has a flow rate ranging between about 3 slm and about 4 slm (standard liter per minute).

In some embodiments, the cap layer 224 is removed in the etching process to expose the underlying strained material 222. Next, in an embodiment, a treated strained material 222′ is formed in the recess cavity 220 by redistributing at least a portion of the corner 222 d of the strained material 222 to be located in the recess cavity 220. The redistribution may increase an amount of the strained material 222 in the recess cavity 220, therefore fabricate a large-volume strained structure to enhance carrier mobility and upgrade the device performance of the semiconductor device 200. In an alternative embodiment, the corner 222 d above the surface 202 s of the substrate 202 is redistributed to be completely located in the recess cavity 220, therefore all the treated strained material 222′ is within the recess cavity 220. Meanwhile, there is not a redistribution process performed on the strained material 222 in the recess cavities 230, 240.

In one embodiment, the redistribution results due to reducing the high surface energy of the tip in corner 222 d. In an alternative embodiment, the redistribution results due to a reflow process in the etching process. The original upper surface 222 b of the strained material 222 in the recess cavity 220 is transformed into a treated upper surface 222 b′ after the etching process. In some embodiments, the treated upper surface 222 b′ has a transformed crystal plane deviated from the original (111) crystal plane, therefore, the lower sidewall surface 222 c of the treated strained material 222′ is not parallel to the treated upper surface 222 b′. In the present embodiment, the treated upper surface 222 b′ has a (311) crystal plane. In FIG. 7A, the semiconductor device 200 is enlarged for better understanding of the profile of the treated strained material 222′. As shown in FIG. 7A, the normal line L1 is perpendicular to the surface 202 s of the substrate 202, a normal line L2′ is perpendicular to the treated upper surface 222 b′ of the treated strained material 222′ in the recess cavity 220, and an angle θ₄ is between the normal line L1 and the normal line L2′. In one embodiment, the angle θ₄ is less than the angle θ₃. In another embodiment, the angle θ₄ is about 25-35 degrees.

Referring to FIGS. 1 and 8, the method 100 continues with step 114 in which contact features 226 are formed over the treated strained material 222′ in the recess cavities 220, 250 and the strained material 222 in the recess cavities 230, 240. In the present embodiment, the contact features 226 are formed by a process that is the same as the process for forming the strained material 222 or the cap layer 224. The contact features 226 may provide a low contact resistance between the cap layer 224 and a silicide structure formed subsequently. In at least one embodiment, the contact features 226 have a thickness ranging from about 150 Angstroms to about 200 Angstroms. In some embodiments, the contact features 226 comprise a material that is the same as the material of cap layer 224. In alternative embodiments, the contact features 226 comprise a material that is the same as the material of strain material 222.

It is understood that the semiconductor device 200 may undergo further CMOS processes to form various features such as contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. In some embodiment, the gate stacks 210 a, 210 b, and 210 c may be dummy gate stacks. Thus, the CMOS processes further comprise a “gate last” process to replace the polysilicon gate electrode with a metal gate electrode to improve device performance. In one embodiment, the metal gate electrode may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other suitable conductive materials, or combinations thereof. It has been observed that the modified strained structure provides a given amount of strain into channel region of a semiconductor device, thereby enhancing the device performance.

The various embodiments of the present disclosure discussed above offer advantages over conventional methods, it being understood that no particular advantage is required for all embodiments, and that different embodiments may offer different advantages. One of the advantages is that the lower portion of the strained material in the S/D recess cavity adjacent to the isolation structure may be increased to enhance carrier mobility and upgrade the device performance. Another advantage is that the likelihood of device instability and/or device failure that results due to forming a subsequent silicide over the lower portion of the strained material may be prevented.

In one embodiment, a method for fabricating a semiconductor device includes forming an isolation feature in a substrate, forming a gate stack over the substrate, forming a source/drain (S/D) recess cavity in the substrate, where the S/D recess cavity is positioned between the gate stack and the isolation feature. The method further includes forming an epitaxial (epi) material in the S/D recess cavity, where the epi material has an upper surface which including a first crystal plane. Additionally, the method includes performing a redistribution process to the epi material in the S/D recess cavity using a chlorine-containing gas, where the first crystal plane is transformed to a second crystal plane after the redistribution.

In another embodiment, a semiconductor device includes a gate stack over a substrate, an isolation feature in the substrate, a recess cavity in the substrate between the gate stack and the isolation feature. Additionally, the semiconductor device includes an epitaxial (epi) material in the recess cavity, where the epi material has a first crystal plane at a bottom surface of the recess cavity. Furthermore, the semiconductor device includes a second crystal plane at a top surface of the recess cavity, where the first crystal plane is different from the second crystal plane. The semiconductor device further includes a contact feature over the epi material.

In still another embodiment, a semiconductor device includes a gate stack over a substrate, an isolation feature in the substrate, a recess cavity in the substrate between the gate stack and the isolation feature. Additionally, the semiconductor device includes an epitaxial (epi) material in the recess cavity, where the epi material includes a (111) crystal plane and a (311) crystal plane.

While the disclosure has described by way of example and in terms of the preferred embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A semiconductor device comprising: a gate stack over a substrate; an isolation feature in the substrate; a recess cavity in the substrate between the gate stack and the isolation feature; an epitaxial (epi) material in the recess cavity, the epi material has a first crystal plane at a side surface connecting to a bottom surface and a second crystal plane at an upper surface contacting the isolation feature, the bottom surface being substantially parallel to a top surface of the substrate, wherein the first crystal plane is different from the second crystal plane; and a contact feature over the epi material, wherein the contact feature extends along a sidewall of the isolation feature.
 2. The semiconductor device of claim 1, wherein the substrate comprises a silicon-on-insulator structure.
 3. The semiconductor device of claim 1, wherein the isolation feature comprises at least one of silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or a low-K dielectric material.
 4. The semiconductor device of claim 1, wherein the gate stack comprises a gate electrode layer.
 5. The semiconductor device of claim 4, wherein the gate electrode layer has a thickness ranging from 30 nanometers (nm) to 60 nm.
 6. The semiconductor device of claim 1 further comprising a hard mask layer over the gate stack.
 7. The semiconductor device of claim 6, wherein the hard mask layer has a thickness ranging from 100 Angstroms (Å) to 800 Å.
 8. The semiconductor device of claim 1 further comprising gate spacers on opposite sidewalls of the gate stacks.
 9. The semiconductor device of claim 8, wherein the gate spacers have a thickness ranging from 5 nm to 15 nm.
 10. A semiconductor device comprising: a gate stack over a substrate; an isolation feature in the substrate; a recess cavity in the substrate between the gate stack and the isolation feature; an epitaxial (epi) material in the recess cavity, wherein the epi material has a first crystal plane at a side surface directly connected to a bottom surface of the recess cavity and a second crystal plane at an upper surface adjacent to the isolation feature, and the epi material extends along a portion of a sidewall of the isolation feature; and a contact feature over the epi material.
 11. The semiconductor device of claim 10, wherein the contact feature contacts the sidewall of the isolation feature.
 12. The semiconductor device of claim 11, wherein the contact feature has a thickness ranging from 1 nm to 5 nm.
 13. The semiconductor device of claim 11, wherein the contact feature comprises a material different from the epi material.
 14. The semiconductor device of claim 10, wherein a vertical distance from an uppermost region of the substrate to a lowest region of the recess cavity ranges from 20 nm to 70 nm.
 15. The semiconductor device of claim 10, wherein the epi material has a (311) crystal plane over the first crystal plane.
 16. A semiconductor device comprising: a gate stack over a substrate; an isolation feature in the substrate; a recess cavity in the substrate between the gate stack and the isolation feature; an epitaxial (epi) material in the recess cavity, wherein the epi material has a first crystal plane at a side surface directly connected to a bottom surface of the recess cavity and a second crystal plane at an upper surface connected to the isolation feature of the recess cavity, and the epi material includes a surface substantially parallel to a sidewall of the isolation feature; and a contact feature over the epi material.
 17. The semiconductor device of claim 16, wherein the first crystal plane is a (111) crystal plane.
 18. The semiconductor device of claim 16, wherein the contact feature contacts a sidewall of the isolation feature.
 19. The semiconductor device of claim 18, wherein the contact feature comprises a material different from the epi material.
 20. The semiconductor structure of claim 16, wherein the epi material further comprises a (100) crystal plane. 